At present, photofunctional devices such as LED, LD, CCD, photodiodes, and photocouplers are generally encapsulated with ceramic packages and transparent plastics. One promising material for encapsulating these devices is an epoxy molding compound in the form of a filled epoxy resin.
The fillers to be used with epoxy resins are required to have not only a high light transmittance, but also a high refractive index approximately equal to that of epoxy resins for deterring any loss of transmittance due to light scattering. By blending fillers having high transmittance and high refractive index in transparent epoxy resins, there can be obtained epoxy molding compounds which are fully transparent. Nevertheless, such fillers fulfilling both high transmittance and high refractive index have never been available in the art. There is a need for the development of such fillers.
For the protection of sophisticated electronic parts such as semiconductor devices, they were generally encapsulated with epoxy resins. The epoxy resins for this application should have excellent properties including low shrinkage, low expansion, heat resistance, and moisture resistance. In encapsulating semiconductor devices with epoxy resins, low shrinkage and low expansion are important in preventing resin cracking and element breakage. Typical prior art attempt for improving the shrinkage and expansion of epoxy resins is by blending inorganic fillers therein.
In encapsulating optical semiconductor devices such as light emitting and receiving elements with transparent epoxy resins, however, no fillers are blended in the epoxy resins so as not to detract from their transparency. Filler-free epoxy resins have a high shrinkage factor and a high coefficient of expansion when cured. Then the problems of resin cracking and element breakage arise, particularly when large-size light emitting elements are encapsulated. It was proposed to blend special fillers in epoxy resins to form transparent epoxy resin compositions. The prior art epoxy resin compositions suffered from the problem that attempts to prevent resin cracking and element breakage can reduce the transparency of epoxy resins, thus adversely affecting the optical function of elements whereas attempts to maintain transparency are not effective for preventing resin cracking and element breakage. There is a need for an epoxy resin composition capable of meeting both transparency and low stress.
In turn, it is well known in the art to prepare titania-silica (TiO.sub.2 -SiO.sub.2) glass by hydrolyzing and polycondensing a silicon alkoxide and a titanium alkoxide in organic solvent to form a TiO.sub.2 -SiO.sub.2 sol, and causing the sol to gel, followed by drying and sintering. This method is generally known as sol-gel method. The TiO.sub.2 -SiO.sub.2 glass prepared by the sol-gel method is characterized by a high refractive index and a low coefficient of thermal expansion and free of impurities such as alkali metals, alkaline earth metals and chloride ions, and these features suggest potential application in a variety of uses.
However, the prior art sol-gel methods were not successful in preparing TiO.sub.2 -SiO.sub.2 glass beads having a high content of TiO.sub.2 and high transparency in the visible to near infrared range. Also, no attempts were made in the prior art for increasing the transparency of TiO.sub.2 -SiO.sub.2 beads.
The inventors proposed in Japanese Patent Application No. 272643/1988 a method for preparing a TiO.sub.2 -SiO.sub.2 glass which shows no absorption in the wavelength range of from 400 nm to 1300 nm, is colorless and transparent, and has a high TiO.sub.2 content and a refractive index (n.sub.D) of 1.53 or higher. Although this TiO.sub.2 -SiO.sub.2 glass is colorless and transparent in outside appearance, beads obtained by grinding the glass undesirably have low light transmittance values when measured for linear transmittance by a measurement method to be defined later. The beads are thus less suitable for use as a filler in molding compounds for encapsulating photofunctional devices.